This invention relates to a satellite communication system for effectively carrying out data distribution using a wide-area characteristic, a broadcasting characteristic, and a broad-band characteristic in satellite communications.
In recent years, drastic growth has been observed in a system for carrying out data distribution using the Internet. Not only distribution of music or news, but also Internet schools or colleges for providing students with home education has been popular.
In the manner which will later be described in conjunction with FIG. 1, a conventional data distribution system comprises a plurality of members or subscribers, a plurality of data makers, the Internet serving as a ground communication network connected to the members and the data makers, and a data distribution center connected to the Internet. The data distribution center provides a home page (HP).
In addition, similar data distribution systems or broadcasting colleges using satellite communications are also widely used. For example, a Perfect PC is implemented as such a data distribution system using the satellite communications.
Whether the conventional data distribution system is the data distribution system using the Internet or the data distribution system using the satellite communications, the conventional data distribution system instantly distributes data in response to a data request signal from any user.
With the enlargement of a business's area using the Internet, an amount of data transmitted drastically increases. Under the circumstances, large problems have been presented in the above-mentioned data distribution system as follows.
A first problem is that there is a right restriction in quantity for provided data.
For example, although the data distribution system using the satellite communications usually uses a bandwidth of several megahelts (MHz), a data communication capacity per person degrades justly when calls are congested.
On the other hand, inasmuch as the data distribution system using the Internet of the ground communication network transmits a signal having a packet format via a lot of routers, a communication capacity per call degrades in the similar manner when calls are congested. In particularly, packet communications have a bad efficiency and take much time in order to transmit a lot of data.
A second problem is that communications are expensive. This is because the satellite communications are expensive naturally. In addition, inasmuch as the Internet transmits a signal via a number of routers, to transmit a lot of data takes data.
More specifically, the Internet originally is a system for connecting two points with a packet communication system and therefor a communication cost per subscriber is not degraded although the number of subscribers increases. For instance, description will be exemplified in a case of education as follows. It will be assumed that an amount of data is 10 megabytes (MByte) required to a lecture for one hour. If data is read out at a channel speed of 50 Kb/s, time of 1600 seconds or thirty minuets takes. A communication cost therefore reaches a large amount and about one hour takes for reading teaching materials if the subscriber takes two subjects for one day. In addition, when the number of the subjects increases, time and cost takes for distributing the teaching materials and it results in difficulty in realizing the business.
A third problem is that it is difficult to communicate in an interactive mode of a momentary fashion. This is because, in the manner which is described above, the Internet has a characteristic where data is always stored in a home page acting as a center and any user reads the data out of the home page via a communication line in necessary. In order to carrying out a seminar at the interactive mode of the momentary fashion, it is necessary to establish a telephone line between the subscriber or a receiver and the data maker or a sender and it is difficult to carry out this when the number of participants increases.